System A Modulator: Options Today

If you like discrete circuitry then Darius' modulator design is delightfully stable. I used it when I built a converter based on his and am using it again to operate the graphics card.

Peter

  Are yes! I forgot about Darius' one  ? 

Posted by: Chris 

I believe David Grant made one, perhaps Catho can comment and I'm sure there was a photo over on Vrat 1.0 but I can't find it for now. 

Ah-ha... My mistake, the 'Li'l Dino' modulator was discussed here on Vrat 2.0 see here

Posted by: Chris

Posted by: Chris 

I believe David Grant made one, perhaps Catho can comment and I'm sure there was a photo over on Vrat 1.0 but I can't find it for now. 

Ah-ha... My mistake, the 'Li'l Dino' modulator was discussed here on Vrat 2.0 see here

Yes, and it's no longer in production.

I remember seeing a load of boards for the ETF modulator when I was at Brian Cuff's reassembling some of his pre-war TV projects ready for sale.

Posted by: Chris

Then there's the one by David Looser as detailed on Jon's Valve page ( See Here) without checking not sure if the parts are still readily available, did anyone build one?

 

They're pretty much stock semiconductors but the biggest problem these days are the formers for coils - although I have seen them on eBay!

One System A modulator not so far mentioned in this thread is the one developed by David Robinson over at UKVRRR.

You can find the discussion here but a word of caution: ignore links to his ntlworld.com pages as they no longer work but the all important schematic can be found in pdf form in Post 33 (there is an earlier one in Post 22 but this doesn't include the 3.5MHz trap in the video input.

David's design uses the same modulator chips as the Aurora, although I think it predates that. Like the Aurora, the output is switch adjustable to any channel between B1 and B13.

Thanks for that post Terry I was completely unware of that thread  ? 

Where possible and data exists to capture these cct designs, I've created a new section in the Data Library called Modulators. 

Has anyone tried this circuit, I downloaded  it quite a few years ago but cannot remember where from!

Hi Dave,

that's Steve McVoy's one from the ETF museum, the one I mention ( see paragraph 2) and provide the link to in post #1. The data is also in the library here.

Hi Chris,

               I even bought 4  Mc1374 to build the circuit. Did the one you have perform OK? I may get around again to building it.

I’ve got one of the ETF modulators here, still incomplete! I never got very far with it as I ended up buying an Aurora instead. I really should finish it as I bought a lovely little anodised aluminium box for it, and it’d be a shame not to use it! 

Regards,

Lloyd

I use a ETF modulator along side an Aurora so I can have London and Birmingham (ch1 & ch4) at the same time. I have to say the Aurora is the best choice but the ETF does work well.

If anybody has a copy of Carnt & Townsend's 1961 book 'Colour television. N.T.S.C. system, principles and practice' you should be able to find a valved solution!

It's a long time since I last saw a copy (around 50 years!) but, from memory it shows the circuit of the low level stages of the Crystal Palace transmitter. I recall it used GEC valves.

The reason these diagrams were included, I think, was because some slight modification to System A was required for colour working - in particular, a slight change to the sound and vision spacing to get the correct relationship between the vision, colour subcarrier and sound frequencies to ensure that the sound-colour beat was stationary on the screen. Ignore the PA stages (which aren't shown, anyway) and you have a System A modulator!

I have just been looking at the David Robinsons System A modulator. A copy of the circuit is in the library. The PIC 16c84 chip would need to be programmed is a copy of that file available?

Hi Dave, 

sorry no.

In post #26 of the UKVRRR thread David Robinson states if asked he can supply the pic file. However the thread is 13 years old and is closed. I believe you can contact the moderators over on UKVRRR stating you would like the thread re-opened and why, or you could just try pm'ing David Robinson and ask directly.

Hi Chris, I think I will have a go at building S. McVoy's version. I have a vcr that I have recorded 405 line material from an Aurora. If I build the modulator I could fit it in the vcr and get 2 channels working with different programmes at the same time! Thanks again.

Posted by: abctelevision

Hi Chris, I think I will have a go at building S. McVoy's version. 

Excellent, if you get the time and feel so inclined, perhaps you might share the process of the build in the workshop section of the forum. I'm sure the membership would find it an interesting project to follow.

I keep fancying making a tunable one using a couple of AD9850 DDS synthesisers. I might get round to it one day !

A bit late to this thread but if anyone is interested I have built a couple versions of David Robinson's modulator. They have a inbuilt 405 line test card and test tone.
All files required to build the latest version can be downloaded here.

Frank

https://www.pcbway.com/project/shareproject/VOL_System_A_Modulator.html